1. Field of the Invention
This invention relates to pincushion distortion correction apparatus and, more particularly, to an apparatus to this end having a control coil and a controlled coil, these coils being coupled to a saturable magnetic core in a perpendicular relation to each other.
2. Description of the Prior Art
A prior-art saturable reactor apparatus for correcting pincushion distortion in a television receiver has a construction as shown in FIG. 1.
In the pincushion distortion correction transformer 21 shown in FIG. 1, saturable E and I type cores 22 and 23 are held in face to face relationship to each other with a predetermined gap l.sub.g provided between them, a control coil 27 is wound on a central leg 24.sub.A of the E type core 22, and first and second controlled coils 28.sub.B and 28.sub.C are wound on the respective opposite end legs 24.sub.B and 24.sub.C of the same core such that they operate differentially with respect to each other. For correcting horizontal pincushion distortions, a parabolic current changing at the vertical scanning frequency is caused through the control coil 27 to modulate the horizontal deflecting current flowing through the controlled coils 28.sub.B and 28.sub.C, thus obtaining a correction such that the horizontal deflection current becomes maximum at the center of the vertical scanning portion. The inductance L.sub.H of the transformer 21 is given as ##EQU1## where N.sub.H1 and N.sub.H2 are respectively turns numbers of the first and second controlled coils 28.sub.B and 28.sub.C (N.sub.H1 =N.sub.H2), l is the average length of the magnetic path, S is the sectional area of the core, and .mu..sub.e is the effective magnetic permeability while .mu.O is the magnetic permeability of a vacuum.
FIG. 2 shows a pincushion distortion correction transformer disclosed in a earlier patent application filed by the same applicant, and it is mentioned here for the purpose of faccilitating the understanding of the present invention. In this transformer 31, a control coil 37 and a controlled coil 38 are wound in a perpendicular relation to each other on legs 34 and 35 of a pair of four leg cores 32 and 33. The core 32 has a square or rectangular plate-like base portion 34.sub.E and four legs 34.sub.A to 34.sub.D extending from the four corners of the base portion. The four legs have equal sectional area. Likewise, the core 33 has a base portion 35.sub.E and four legs 35.sub.A to 35.sub.D. In FIG. 2, the legs 34.sub.D and 35.sub.D are concealed and not shown. The cores 32 and 33 having this construction are held such that the ends of the legs 34.sub.A to 34.sub.D of the former are respectively brought into contact with the corresponding legs 35.sub.A to 35.sub.D of the latter through predetermined gaps to define a constant space l.sub.g. The control coil 37 is wound on the legs 34.sub.B and 34.sub.D of the core 32 as a set, and the controlled coil 38 is wound on the legs 35.sub.A and 35.sub.B of the core 33 as a set. The inductance of this perpendicular transformer 31 is given as ##EQU2## N.sub.H in equation (2) is the turns number of the controlled coil 38 having almost half the turns of FIG. 1, and this means that for the example in FIG. 2 substantially the same variable inductance characteristic can be obtained with one half the turns number compared with the prior-art example of FIG. 1.
However, these pincushion distortion correction saturable reactor apparatus shown in FIGS. 1 and 2, have disadvantages; for example, raster shrinkage occurs in the neighborhood of the center of the screen, and the horizontal linearity is inferior.
FIG. 3 is a graph showing the variable inductance characteristic of the perpendicular transformer 31 shown in FIG. 2. In this graph, the abscissa represents the horizontal deflecting current I.sub.H (in A), and the ordinate represents the inductance L.sub.H (in .mu.H). The vertical deflecting current I.sub.V (in mA) is taken as the parameter, and characteristic curves for 0,10,20,40 mA respectively are shown. When the inductance characteristic is as shown in FIG. 3, the reproduction on the screen has a character as shown in FIG. 4. The reproduction shown in FIG. 4 is obtained when a reference pattern consisting of a plurality of uniformly spaced vertical lines is reproduced on the television screen after the horizontal pincushion distortion correction using the transformer 31 mentioned above. The ratio of the interline space in the reproduction of FIG. 4 with respect to the interline average space in the reference pattern (in %) is as shown in FIG. 5. It will be seen that the raster shrinkage is produced in the neighborhood of the center of the screen, particularly in a horizontal deflecting current region between from -1 to +1 A. Due to this shrinkage, the horizontal linearity is inferior. Therefore the dynamic range of the variable inductance defined by the DC superimposition characteristics thereof becomes narrow.